The present invention relates to a compound semiconductor FET (Field Effect Transistor) and electronic circuits using the same, and more particularly, to a GaN semiconductor FET.
Conventionally, there have been known GaN heterojunction FETs (GaN HFETs). As an example, a GaN heterojunction FET disclosed in U.S. Pat. No. 5,192,987 will be described with reference to FIG. 7. Semiconductor layers of FIG. 7 are of the hexagonal C-plane Ga-surface type. In the semiconductor layers and metal layers of FIG. 7, reference numeral 21 denotes a sapphire substrate, 22 denotes a 100 nm thick undoped AlN layer, 23 denotes a 1 μm thick undoped GaN layer, 24 denotes a 30 nm thick undoped Al0.2Ga0.8N layer, 25 denotes a Ti/Al/Pt/Au source ohmic electrode, 26 denotes a Pt/Au gate Schottky electrode, and 27 denotes a Ti/Al/Pt/Au drain ohmic electrode.
A channel is generated at an interface between the undoped GaN layer 23 and the Al0.2Ga0.8N layer 24, causing a two-dimensional electron gas (2DEG) of electrons to be generated. The concentration of this 2DEG is controlled with a bias voltage applied to the Pt/Au gate Schottky electrode. When the applied bias voltage goes beyond the “pinchoff voltage Vp” of the device, the concentration of the 2DEG becomes zero.
However, conventional GaN HFETs have a problem that even if the gate voltage goes beyond the pinchoff voltage Vp with the result of a zero concentration of 2DEG, there would remain electric charges under the gate so that application of a voltage to between source and drain would cause a current flow. This phenomenon is called as “parallel conduction.” Referring to DC characteristics (drain current-drain voltage characteristics) of a conventional GaN HFET shown in FIG. 10, because of a low output resistance, there occurs no pinchoff even when a bias voltage Vg has reached the pinchoff voltage (Vp=−4 V).
Another problem of GaN HFETs is that when the gate voltage goes beyond the pinchoff Vp, there would flow a gate leak current. This phenomenon is called as “gate leak”. Referring to gate-source Schottky diode characteristics of conventional GaN HFETs shown in FIG. 11, there can be recognized a characteristic that the leak current linearly increases as the reverse bias voltage of the gate increases above a certain voltage.
The smaller the thickness of the GaN layer 23 is, the more noticeable these problems of “pinchoff” and “gate leak” become.